Additional temporary water storage capacity is required near the confluence of the Ohio and Mississippi rivers to handle peak flows and to avoid the flooding that occurs during extended periods of above-average rainfall, say University of Illinois (UI) researchers.

UI researcher Ken Olson and colleagues are studying the levees, diversions and floodways built over the past 200 years that have allowed land conversion from wetlands to agriculture in southeast Missouri. He says these pieces of infrastructure have "substantively altered the hydrologic cycle of the region.”

(Read “How Engineers Can Adapt Infrastructure Design for a Changing Climate” and “Hardening the Infrastructure: Flood Management Controls.”)

A headwater diversion channel. Image credit: University of Illinois.A headwater diversion channel. Image credit: University of Illinois.According to Olson, the Little River levee and Little River Drainage District Headwaters Diversion channel, built in the 1910s, permitted draining the 1.2-million-acre Big Swamp in southeast Missouri. However, it also had the unintended consequence of increasing the peak flow of Mississippi River water south of Cape Girardeau through the Thebes gap and south to Helena, Ark., a distance of 360 river miles, he says.

“When the Ozark uplands and Francois Mountains experience above-average rainfall for extended periods of time, the additional runoff transported by the diversion channel increases the chances of Mississippi River levee breaches south of Commerce, Missouri, and adds to the peak river height at the confluence of the Ohio and Mississippi rivers,” he says.

Olson says the increase in Mississippi River peak flow placed additional river pressure on levees and led to increased flooding, especially during the floods of 1927, 1937, 2011 and 2015-2016.

“The Kentucky, Illinois and Missouri farmers’ and landowners’ response to the additional volume and height of the Mississippi River from the diversion channel valley, and the prevention of the Mississippi River floodwaters from flowing into the ancient Mississippi River valley and Big Swamp, was to build floodwalls and levees,” Olson says.

After a 1915 flood, Cape Girardeau built a floodwall to protect the city. Likewise, after the Great Flood of 1927, Cairo built a floodwall, strengthened levees and created the Birds Point-New Madrid floodway. Missouri farmers built the Commerce Farmer levee, which failed in 2011. Kentucky farmers built the Hickman levee—strengthened later by the U.S. Army Corps of Engineers—which has not failed.

“Illinois farmers built the Fayville-Len Small levee that breached in 1993 and 2011,” Olson says. It breached a third time on Jan. 2, 2016, when the Thebes river gauge reached a record 47.7 feet –14.7 feet above flood stage.

Olson says climate scientists predict a continued pattern of extreme rainfall events in the upper Mississippi River region. “This suggests that unexpected above-average rainfall events in the Ohio and Mississippi River basins will continue to increase the frequency of extreme flooding events on these great rivers.”

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